Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long life span and low self-discharge are commercially available and widely used.
The lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like as the anode active material are considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle life, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life.
In recent years, methods to use lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a NASICON structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known NASICON structure compound, and LiFePO4 and Li(Mn,Fe)PO4 are the most widely known olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high voltage of 3.5 V and a high bulk density of 3.6 g/cm3, as compared to lithium, has a theoretical capacity of 170 mAh/g and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe, thus being highly applicable as the cathode active material for lithium secondary batteries.
However, LiFePO4 has limited practical application due to the following disadvantages.
First, LiFePO4 exhibits low electrical conductivity, thus disadvantageously causing an increase in inner resistance of batteries, when used as the cathode active material. This increase also leads to an increase in polarization potential, when electric circuits close, and thus a decrease in battery capacity.
Second, LiFePO4 has a density lower than that of a general cathode active material, thus having a limitation in that considerably increasing the energy density of batteries is not possible.
Third, since an olivine crystal structure, from which lithium is deintercalated, is extremely unstable, a passage, allowing the lithium to escape from the crystal structure is blocked and intercalation/deintercalation rate of the lithium is thus delayed.
In this regard, Japanese Patent Application Publication No. 2001-110414 discloses incorporation of a conductive material into olivine-type metal phosphate to improve conductivity and Japanese Patent Publication No. 2001-85010 discloses a technology for doping electrochemically stable elements to stabilize crystal structure.
However, these technologies relatively deteriorate a volume rate of a cathode active material in batteries, thus lowering an energy density of batteries. For this reason, these technologies cannot provide an ultimate solution. In addition, addition of a conductive material or doping elements inevitably entails an addition or substitution process, thus disadvantageously increasing manufacturing costs and deteriorating process efficiency.
In response to this, a decrease in size of olivine crystals to a nanometer-scale in order to shorten a movement distance of lithium ions and thus increase discharge capacity is disclosed in Japanese Patent Application Publication Nos. 2002-15735 and 2004-259470.
However, fabrication of electrodes using such an olivine particle with a fine diameter inevitably entails use of a large amount of binder, thus disadvantageously lengthening slurry mixing time and deteriorating process efficiency.
Accordingly, there is an increasing need for lithium iron phosphate such as LiFePO4 that exhibits superior electrical conductivity and density as well as process efficiency.